Telecommunication switching system



April 29, 1947.

G. DEAKIN TELECOMMUNIGATION SWITCHING SYSTEM Filed Aug. 28, 1943 lNvx-:NToR i GERALD 05AM/V ATTORNEY Patented Apr. 29, 1947 TELECOMMUNICATION SWITCHING SYSTEM Gerald Deakin, New York, N. Y., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application AugustZS, 1943, Serial' No. 500,411

22 Claims. l

This invention relates to new vand useful improvements in telecommunication switching systems and more particularly to selector control circuits for such systems.

The Vobject of the invention is accurately and rapidly to vcontrol a switch in the performance of a selective operation, e. g. the seizure of a line.

My co-pending application, Serial No. 484,184, led April 23, 1943, now Patent No. 2,380,950, dated Aug. 7, 1945, and the co-pending application of Henry F. Herbig, Serial No. 485,827, filed MayfS, 1'943, now Patent No. 2,354,682, dated Aug. 1 1944, disclose telephone systems in which a gas valve ina register controller fires when a selector switch tests a free line. A relay will then operate to removeground from certain relays in the register 4and Vfrom a relay and a Yclutch magnet in the selector. Thus the 4selector is stopped and upon the de-energization of th'e selector relay a busy- 'ing potential is placed on the test terminals through which the trunk or lines are accessible. From the moment the idle line is encountered until it is made busy, a period of 4,000-8,000 microseconds elapses. Short as this delay is, it

will sometimes happen that a gas valve controlling a second selector will re during this period land lcause the seizure of the same line by two selectors.

Further 'to guarantee against double connections, it has been the Vpractice to ymake th'e test twice. Such double test may be made, for instance, by two electromagnetic test relays, first over one brush and then over a second brush of the selector. However, the twin test relays are always tested for a fixed condition. While the arrangement is effective against double connections, it does not yaiford protection yagainstoversteps which may result in wrong or double connections.

According to the present invention, means are providedmaterially to .reduce the period between the above-mentioned testing and energizing conditions of a relay. In the exemplication of the invention hereinafter disclosed this delay is reduced to 200-100 microseconds. In a time so short th'e 'possibility of double testing is, for all practical purposes, eliminated.

'I accomplish this by using electronic means such as va `gas valve or Yother electronic switch for testing the line. In accordance with the present embodiment of the invention, the gas valve rst tests twice, preferably first over one 'and then overa second brush of the selector switch.

A trunkmayl become free just as the selector test brush is about to leave the test terminal and the selector may voverstep to a busy vter-mlnal while there is still time to fire the gas valve. To guard against'this, according to another feature of the present invention, the second test is always made for the same potentials as the first test so as to prevent seizure of the selected terminals and stop the progress of the connection if the second test does not iind the same condition as the `iirst test. Should the selector overstep to a free trunk to which-the same potential is applied yas to the previous trunk, then that trunk will be seized and the connection will'proceed in the usual way. Should, however, the overstep be to a Vfree trunk having -a diierent potential, Aor to a busy trunk or to the space between two terminals, then establishment of the connection will be discontinued.

"The `invention will Anow Abe explained with the Yaid 0f the drawing which diagrammatically illustrates as much of an automatic telephone system register and a selector controlled thereby as'is necessary 'for an understanding of th'e invention.

It is assumed that a calling line Vhas been connected to a second selector and that the register has been set by the 'calling subscriber to control the numerical operation of the second selector.

As described in my co-'pending application, a register switch ED of which only one terminal bank is shown advances from-position 1 to position 2 when dialing commences and in so doing will cause a relay RSR'in the register to operate. This will connect ground at the armature of relay GVR over the'back contact of this relay and the right-hand armature and front contact of RSR to the a wire connected over the first kgroup selector to the A talking brush of lthesecondgroup selector. Relay AR in the second selector 'is connected with this wire over the normally closed contact ONCE. The relay becomes energized and locks up over its 'left-hand front contact andar.- mature 3, and connects clutch magnet P 'of the selector over its armature 4 and front contact in parallel 'with its own locking circuit.

Over its armature l and, back contact the relay .AR disconnects the b wire from the normal test potential --'N which may be conneeted'to it over an off-normal Contact ONC4 and the armature I and front contact of relay BR. The b wire'will now be connected over the 'armature l and front contact of AR, and armature Zand front contact of relay BR to the test brush T ofthe selector. The relay BR was energized and locked vup immediately upon the seizure of the second group selector by the rst group selector over a circuit including the armature 4 and iront contact of the BR relay associated with the rst selector (which is like the second selector) resistance IR2 in the second selector, the c wire and ONCZ.

At this stage, a slow releasing relay RSR2 in the register which was energized by RSR connects the b wire to the cathode of a triode VI and the grid of a triode V2. This slight delay is introduced to prevent a premature and false second test to the test potential N which would occur if relay AR in the selector failed to break the back Contact of armature I before RSR2 in the register 2 made its front contact.

Assuming that the calling subscriber has set the register switch brush RB to a terminal to which -N potential is connected, when owing to the energizing of the clutch magnet P the selector brush T is moved into engagement with a free terminal having the same potential, the gas valve GV will be iired and will cause the operation of relay GVR. This is so because when the same potentials are applied to the electrodes of the two triodes, the potential applied over resistances R3 and R5 from the high tension battery to control electrode CE2 will be such as to bring about the firing of the gas valve, provided that normal test potential is applied to the other control electrode CEI. The operating circuit of GVR may be traced from a source of potential -N in the third group selector (identical with the first and second selectors), a resistance IRT, the ONC!! contact of the third group selector, the armature I and back contact of the BR relay of this selector. terminal and test brush T of the second selector. front contact and armature 2 of BR. front contact and armature I of AR. the b wire, armature and front contact RSRZ, the control electrode CEI and main gap electrode MG of gas valve GV. winding of GVR. the back contact and armature of a relay GSR to high tension battery. The instant GV fires. the potential applied to the T brush and, therefore, to the multiple test terminals with which it is in engagement. is changed on account of the connection of this high tension battery over the ionized path in gas valve GV. The normal test potentials are of the order of -2 volts to 26 volts. When the above traced test circuit is closed. these negative potentials are increased to 50-75 volts depending on the value of resistance IRT. This change of potential makes the selected trunk instantly busy and guards it against seizure by another selector since no other gas valve could nre when such potential is applied to its control electrode CEI. It will be seen, therefore, that GV acts as an electronic switch instantaneously to busy the line seized by the selector.

GVR removes ground from the a wire and deenergizes RSR, AR and the clutch magnet P. AR opens its locking circuit, disconnects the b wire from the T brush. This wire is now connected only to the B brush. However, before the T brush is disconnected from the b Wire, it is connected over the armature 2 and front contact of relay BR, armature 2 and back contact of relay AR to the t wire and therethrough to a ground at the rst group selector. Thus the busy guard on the 'I' terminal is not broken during the transfer.

The de-energization of P will stop the second group selector, whereupon brushes A, B and C make contact with their respective terminals. The b wire of the selected third selector is now connected through to the register. The oil-normal contacts ONCI, 2 and 4 open when the 4 selector leaves its normal position and before it tests the rst terminal. ONC3 closes when the other off-normal contacts open.

In the register, GVR will cause the energization of GSR which opens the high tension battery connection long enough to de-ionize gas valve GV and to release GVR. This relay will open the circuit of GSR, but before the latter opens its front contact it will cause the energization of a slow releasing relay GRR by ground applied over the back contact of GVR. GRR locks up over the front contact of RSR2 which did not have time to de-energize during the brief operation of RSR.

When GSR again makes its back contact, the trunk selected by the second selector is tested for a second time, this time for potential -N over the B brush of the second selector and the b wire. This potential is now applied to the B terminal over armature I and front contact of the third selector BR relay. Normally, the potential found by the B brush is the same as the potential found by the T brush. Thus GV will fire for the second time and cause the operation of GVR which in this case will advance the out control switch EO of the register over a front contact of GRR. The relay GSR will again be energized and cause the de-ionization of GV and the register circuit will be advanced to control the performance of the next selection.

Should two second group selectors test simultaneously during the short interval of microseconds, then the two associated registers will operate simultaneously and the two selectors will stop on the multiple terminals of the same third selector. However, it is highly improbable. that the time constants of the relays of both selectors and registers will be identical. One will busy the B terminal before it is tested by the .othen The gas valve GV employed in the last-mentioned connection will, therefore, not re and when the associated relay RSRZ releases, a circuit will be closed from ground over the back contact of RSR2, the front contact of GRR to a relay DAR. As described in my co-pending application, this causes the release of the register and of the partially established connection and the application of a busy tone to the calling line.

If, for any reason, the second selector oversteps and comes to rest withits brushes between two terminals, then no potential will be found by the B brush and when the second test is made, the connection will be released as just described.

Should the selector overstep and come to rest on a free trunk of the same numerical group, then the same potential -N will be found on the terminal by brush B. The third selector will be seized and the connection will proceed in the normal way.

Should the selector overstep and come to rest with its brushes on the free trunk of a .diierent numerical group, then the potential'encountered by the B brush will be diiierent from N and GV will not fire. The connection will, therefore, be released as previously described.

If the selector oversteps and stops with its brushes on a busy connection, then the potential applied to the b wire will be the same as of the talking circuit at that point. If the talking circuit is open, then the potential will be 48 volts. If it is closed through. a low resistanceline and a sub-station set of 50 ohms, then the potential will be -26 volts. This potential is not used for .trunk selection and thus the second selector will release without disturbing theblusy connection.

tion and the busy condition of each set of terminais, first over the brush cooperating with said one and then over said another terminal, and electromagnetic means in each selector for controlling the seizure of a terminal set by the selector and controlled by the testing means only if the same potentials are successively applied over both terminals.

12. In an automatic telephone exchange system, a plurality of lines, means including a group of selector switches for establishing a talking connection between a calling and a called line, a register controlled in accordance with the designation of the called line for controlling a selector to establish the connection, each of said selectors having a bank of multiple terminals and a set of brushes cooperating therewith, sources of diierent potentials indicating the busy or idle condition of the terminals, testing means associated with the register for testing the condition of each set of terminals, means for connecting said testing means twice in succession with the brushes of the selector under control, and electromagnetic means in each selector controlled by the testing means to seize a set of terminals only if the same potentials are successively applied over the brushes to the testing means.

13. In an automatic telephone exchange system, a plurality of lines, means including a group of selector switches for establishing -a talking connection between a calling and a called line, a register controlled in accordance with the designation of the called line for controlling a selector to establish the connection, each of said selectors having a bank of multiple terminals and a set of talking and test brushes cooperating therewith, sources of different potentials indicating the positions of the terminals in the banks, contacts for connecting said potentials normally with a test terminal and when the set of terminals is seized with a terminal cooperating with a talking brush, testing means associated with the register for testing the location and the busy condition of each set of terminals, rst over the test brush and then over -a talking brush of the selector taken into use, and electromagnetic means in each selector controlled by the testing means to seize a set of terminals only if the same potentials are successively applied over both terminals.

14. In an automatic telephone exchange system, a plurality of lines, means including groups of selector switches for establishing a talkin-g connection between a calling and a called line, a register controlled in accordance with the designation of the called line for successively controlling a selector in each group to establish the connection, each of said selectors having a bank of multiple terminals and a set of talking and test brushes cooperating therewith, sources of different D. CL potentials indicating the positions of the terminals in the banks, contacts in each selector for connecting said potentials normally with a test terminal and when seized with a talking terminal in the preceding multiple bank over which the selector may be seized, testing means associated with the register for testing the location and the busy condition of each set of terminals, rst over the test brush and then over a talking brush of the selector taken into use, and electromagnetic means in each selector controlled by testing means to control the seizure of a set of terminals only if the same potentials are suc- K magnet for controlling the seizure by the brushes of a set of terminals, sources of diierent potentials, connections for applying different potentials to the terminals of a set, a variably operable register for controlling the selector magnet and arranged to select a potential designating the location of a desired terminal, a multi-electrode discharge device, connections from the electrodes of said device to the register and to the selector brushes, means for closing twice in succession the connection between the device and the selector brushes, a circuit for said magnet controlled by said discharge device depending on the relationship of the potentials applied to the connections and means for seizing a set of terminals only if the same potentials are applied over the selector brushes to the device during its rst and second connection therewith.

16. In combination, a selector having a bank of terminal sets and a set of cooperating brushes, a magnet for controlling the setting of the brushes on a set oi terminals, sources of different poten tials, connections for applying different potentials to the terminals cooperating with two brushes, said potentials designating the positions of said terminals in the bank and the busy and idle condition thereof, a v-ariably operable register for controlling the selector magnet and arranged to select a potential designating the location of a desired terminal, a multi-electrode discharge device, connections fromv the electrodesA of said device to the register and to said one of the brushes, a contact in the selector for extending the connection from the discharge device to another brush and a circuit for said magnet controlled by said discharge device depending on the relationship ofthe potenti-als applied to the connections.

17. In combination, a selector having a bank of terminal sets and a set of cooperating brushes, a magnet in the selector for controlling the seizing by the brushes of a set of terminals, sources of different D. C. potentials, connections for applying different potentials to the terminals, said potentials designating the positions of said terminals in the bank and the busy and idle condition thereof, a variably operable register for controlling the selector magnet, said register including a variably operable switching device arranged to select a potential designating the location of a desired terminal set, said register also including a gas valve, having control and main gap electrodes, and a multi-electrode discharge device, connections from the electrodes of said device to the register switch and to one of the brushes, contacts in the selector for twice completing the connection from the discharge device to the brushes, a connection from a control electrode of the gas valve to the last-mentioned connection, said discharge device and gas valve being so adjusted that the latter strikes only when the potentials applied to both connections of the discharge device bear a predetermined relationship to one another and the same potential is applied to the control electrode of the gas valve during both operations of the selector contacts and a circuit for said magnet controlled by said gas valve.

18. In combination, a selector having a bank of terminal sets and a set of cooperating brushes, a magnet in the selector for controlling the setting of the brushes on a set of terminals, sources of different D. C. potentials, connections for applying different potentials to the terminals cooperating with two brushes, said potentials des- -plioation.

5 While in the register rcircuit I have illustrated onlyone gasvalve, `a second valve may be used .in -parallel'th'erewith as disclosed in my said copending application. In this case, however, one `of the control electrodes of the second 'gas valve should'be'connected to the conductor leading to theright-hand front contact of RSR2 instead of to` ground as shown in my said co-pending ap- What is yclaimed is:

l. In combination, a switch having a first and aplurality of second sets of cooperating contacts, a-source of potentials, means'for applying a potential to a contact in each second set, testing `means responsive to the potential so applied, a circuit for connecting the testing means to one contact of the rst set and therethrough to the contact of the second set cooperating therewith and electronic means for instantaneously altering the potential applied tothe contact of said second set to prevent the seizure thereof.

2. In combination, a register, a selector controlled thereby and having a bank of multiple terminals and a set o'brushes cooperating therewith, a source of potentials, means for applying a potential to said terminals to indicate the condition thereof, electronic testing means responsive to a predetermined potential connected to trolled thereby and having a bank of multiple terminals and a set of brushes cooperating therewith, a source of potentials, contacts for applying a potential to a set of terminals to indicate the condition thereof, a gas valve responsive to sa'id potentials, a circuit for connecting the valve to said brushes and therethrough to the set of terminals engaged thereby, a circuit for saidgas valve to ionize it only when a predetermined potential exists on the set of terminals with which it is connected, and a source of potential connected over the gas valve to the terminals engaged by the brushes for altering the potential existing on the terminals upon the ionization of the gas valve.

4. In combination, a register, a selector controlled thereby and having a bank of multiple terminals and a set of brushes cooperating therewith, .a source ofrpotentials, contacts for applying potential to a set of terminals to indicate the condition thereof, an electronic switch responsive to said potentials, a circuit for connecting the electronic switch to said brushes and therethrough to the set of terminals engaged thereby, said switch being adj-usted to operate only when a predetermined potential exists on the terminals with which it is connected, and a circuit for altering the potential existing on a terminal controlled by said electronic switch.

5. In a telecommunication switching system, a selector switch having a test brush and a plurality of multiple test terminals, a source of potentials, means for applying predetermined potential to said test terminals to indicate the condition thereof, a magnet for controlling the movement of said test brush over said test terminals, a relay for controlling the operation of said Amagnet to arrest said brush when it engages a terminal on .which a predetermined potential exists, and anelectronic switch controlling said relay 'fand .I altering 'the potential existing `on 'the test terminal torprevent its seizure.

6. In a telecommunication switching system, a group of -selector switches, each having a test brush and a plurality ofmultiple test terminals, a source of potentials, means for applying .predetermined potentials to said test terminals, depending on the idle or busy condition thereof, a magnet for controlling the movement of said test brush over said test terminals, means for controlling the operation 0i said magnet to arrest brush when it engages an idle test terminal, and -anelectronic switch for applying a different predetermined `busy potential to said test brush when its arrest .is initiated by the actuation of said last-mentioned means.

7. In combination, a iirst switch, a second switch controlled-thereby, having two sets of contact-s, and-means for causing the contacts to `cooperate, for performinga predetermined switchingoperation, a source of potentials arranged to be connected to one set of contacts, testing means responsive to said potentials, means for connecting the testing means twice in succession to the second set of contacts, and means controlled by the testing means lfor allowing said contacts to perform said switching operation only if the potentials which are successively applied bear a predetermined relationship to one another.

8. In combination, a register, a selector controlled thereby and having a bank'of multiple terminals and a set of brushes cooperating therewith, a source of potentials arranged to be connected to sa-idterminals, anelectronic switch responsive to said potentials, means for connecting tbeelectronic switch-twice in succession to said brushes, and 'means controlled by the electronic switch for seizing a set of terminals only if the potentials successively applied thereto 'bear a predetermined relationship to one another.

9. In combination, a register, a selector controlled thereby and having a bank of multiple terminals Vand a set of brushes cooperating therewith, a source of potentials arranged to be connected to said terminals, a gas valve responsive to said potentials, means for operatively connecting the valve twice in succession to said brushes, and means controlled by the valve for seizing a set of terminals only if the same potentials Vare successively applied thereto.

l0. Ink combination, a selector, a register for controlling the selector, said selector having a bank of multiple terminals and a set of brushes cooperating therewith, sources of potentials indicating'the busy and `idle condition of the terminals, contacts for connecting said potentials normally with'one and when seized with another terminal in a set, testing means associated with the register for testing the condition'of each set otterminals, first over the brush cooperating with said one-and-then over said another'terminal, and means in said selector controlled by the testing means to 'seize a setof Iterminals only if the `same potentials are successively applied over both terminals.

l1. In combination, a. 'group of selectors, Aa register rior controlling a selector, `said selector having a bank of multipleterminal sets and a set of. brus-hescooperating therewith, sources of different potentials indicating the'positions of Athe terminals in the banks, contacts for connecting said potentials normally with one-and when seized with anotherterminal in a set, testing means associated Withthe register for'testing 'thelocaignating-t-he positions of said terminals in the bank andthebusy and idle condition thereof, av

variably operable register yfor controlling the selector magnet, said-register including a variably operable switching device-i arranged toselect a potential designating the locationof a desired terminal, said registerfalso including a gas-valve,

havingV control and mainl gap electrodes, and a multi-electrode discharge device, connections fromvtheelectrodes'of said device to the register switch andv to one of thetwo brushes, a contact in thel selector foreXtending-the connection fromV thedischargedevice to the other of the tWo of lines,r means including av selector switch forY establishing-a talking connectionbetweenl a callingfV and' a called line, said selector switchy having a bank-or terminal sets and a set of cooperating brushes, including a rst and a second talking brush and a test brush, a magnet in the selector fo'r controlling Ythe seizure by the brushes of a set of terminals, sourcesof diierent D. C. potentials, connections for applying different potentials to the terminals vcooperating with the test 'brush and one of the talking'brushes, said potentials designating the positions of said terminals in the 'bankr and the busy and idle conditionthereof, a register variably operable in accordance with.the designation of the called line for controlling. the selectorl magnet, said register including aY variably. operable switching device arranged to select a potential designating the locationcf a desired terminal, said register also including a gas valve, having control and main gapelectrod'es, and a multi-electrode discharge device, connections from the electrodes of said f device to the register switch and to said one of the talking brushes of the selector switch, a relay inthe-seicctor for extending the-connection from the discharge device to the test brush, a connection from a control electrode of the gas valve to the last-mentioned connection, said discharge device and gas Valve being so adjusted that the latter strikes only when the same potentials are applied to both connections of the discharge device and an idle potential is applied to the control electrode of the gas valve, and a circuit for said magnet controlled by said gas valve.

20. In atelephone exchange system, a plurality of lines, means including one of a group of selector switches for establishing a talking connection between a calling and a called line, each of said selector switches having a multipled bank of terminal sets and a set of cooperating brushes, including a iirst and second talking brush and a test brush, a magnet in' each selector for controlling the seizure by the brushes of a set of terminals, sources of different D. C. potentials, connections for applying different potentials to the terminals cooperating with the test brush and one of the talking brushes, said potentials designating the positions of said terminals in the bank, a register variably operable in accordance with the designation of the called line for controlling the magnet of the selector taken into use, said register including a variably operable switching devicezarrangedto select a potentialidesignating the location of a desired terminal', said register also including a gas valve, having two* control and main gap-electrodes, and twotriodes, a iirst connection fromA the-gridvofl the rst and the cathodeof the-second triode to the register switch, asecond connection from thecathode of therst and the grid of the secondtriode'tosaid one-oi thek talking brushes ofv the-selector inuse, ali-clay` in-said selector for extendings the second connection yto the test brush, a connection fromY a control electrode of the gas Valve-to the-second connect-ion, aconnection from the-second con-Y trol electrode oithe gas valve to theanodes off thet-wo-triodes, said discharge'device andgas`L valve being soadjusted that the latter strikes-L only when the samepotentials are applied to the rst and second connections of the triodesv and? the brushes oi only one selector engage aset` of terminals, and a circuit for said magnet con trolled by said gas valve.-

21. In a telephonesystem, a pluralityof lines, means including one-of a group ofA selectors for interconnecting calling` and called lines, each ofsaid selectors having a-multipled bank of termi-Y nal sets and a set of `brushes cooperating therewith, includingV a first and a second talking con-A ductor brush and a test brush, a magnet and a relay vfor each selector for controlling the seizure by the brushes of a set of terminals, conductorsI terminals and abrush cooperating therewith,V connections-for applying totheterminals ofthe regi-ster switch potentials corresponding tothosedesignating locations of selector terminals, a

high-tension source, two triodes, agas valve andKY relays in Vsaid register, a connectionfrom thejhiglzrY tensionsource to the anodes of the triodes and-a control electrode of the gas valve, an energizing circuit closed bythe register-relaysforitherelay andthe magnet of the selector taken into use 'overr the conductor leading to the rst talking brush, a connection to a source of potential extending over a back contact of the selector relay and the conductor leading to the second talking brush, a connection extending from the test brush over a back contact of the selector relay to ground and over a front contact of the selector relay to ground and over a front contact of the selector relay to the conductor leading to the second talking brush, a connection from the grid of the rst and the cathode of the second triode to the brush of the register switch, a connection from the grid of the second and the cathode of the rst triode to the conductor leading to the second talking brush, a second control electrode for the gas valve connected with the conductor leading to the second talking brush, and a main gap electrode for the gas valve connected with one end of the winding of one of the register relays controlling the selector relay circuits, the other end of which is connected with the high tension source, said triodes and gas valve being so adjusted that the latter strikes and operates said one register relay when equal potentials are applied to the grid and cathode connections of the triodes and the brushes of only one selector are connnected with the selected set of terminals.

a variablyoperable register for controlling the setting of the brushes of a selector taken into use, sources of different D. C. potentials, means including said relays to apply said potentials normally to the terminals cooperating with the test brush and when the selector is taken into use to the terminal cooperating with the second talking brushes, said potentials designating the location and the idle and busy condition of the sets of terminals, said` register comprising a switchvhaving a row of terminals and a brush cooperating therewith7 connections for applying to the terminals of the register switch potentials corresponding tothose designating locations of selector terminals, a first, a second, a third, a fourth and a fth relay in the register, the second and third relays being slow to release, a high tension source in said register, two triodes and a gas Valve in said register, said gas valve having two control electrodes and a main gap electrode, a connection from the high tension source over a back contact of the fourth relay to the anodes of the triodes and a control electrode of the gas Valve, a circuit for operating the first relay upon the adjustment of the register to control a selector, saidcircuit including back contacts of the third `and fth relays, an energizing circuit closed by the rst relay for said relay and the magnet of the selector taken into use and including a back contact of the rlith and a front contact of the rst relay and the conductor leading to the rst talking brush of said selector, a connection to a source of potential extending over aback Contact of one and the front contact of theother selector relay and the conductor leading to the second talking brush, a connection extending from the test brush over a back contact of said selector relay and a front contact of' the other selector relay to ground and over front contacts of said selector relays to the conductor leading to the second talking brush, a connection from the grid of the rst and the cathode of the second triode tothe brush of the register switch, a connection from the grid of the second and the cathode of the iirst triode over a front contact of the second relay to the conductor leading to the second talking ybrush of the selector taken into use, a connection from the second control electrode to the conductor leading to the second talking brush of said selector, a connectionirom the main gap electrode to one end of the winding of the fifth relay, the other end of which is connected with the high tension source over a back contact of the fourth relay, said triodes and gas Valve being so adjusted that the latter strikes and operates the fifth relay when equal potentials are applied to the grid and cathode connections of the triodes and the brushes of only one selector are connected with a set of terminals, an energizing circuit for the fourth relayrclosed over a front contact of the fifth relay, an energizing circuit for the third relay controlled over a front contact of the fourth and back contact of the fifth relay, and alocking circuit of the third relay controlled over the front contact of the second relay.

GERALD DEAKIN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,903,837 Richardson Apr. 18, 1933 2,048,506 Husta July 21, 1936 2,195,809 Holden Mar. 26, 1940 2,195,317 Martin Mar. 26, 1940 2,138,594 Brofos Nov. 29, 1938 2,291,040 Holden July 28, 1942 1,841,254 Flad Dec. 26, 1933 FOREIGN PATENTS Number Country Date British May 1, 1934 

